1. Field of the Invention
The present invention relates to a clock noise canceling circuit applicable to an image sensor module which is employed in a Code-Division Multiple Access (CDMA) mobile phone. More particularly, the present invention relates to a clock noise canceling circuit which is designed to eliminate a harmonic component of a preset frequency from a clock signal of e.g., an image sensor, thereby rendering other parts or signals less susceptible to the harmonic component of the clock signal, and can be substituted for an electromagnetic (EMI) filter.
2. Description of the Related Art
In general, a CDMA mobile phone features a camera function besides a telephone function, which will be explained with reference to FIG. 1.
FIG. 1 is a configuration view illustrating a conventional CDMA mobile phone.
Referring to FIG. 1, the conventional CDMA mobile phone includes a controller 10, a telephone transmitter/receiver 20 and an image sensor module (ISM) 30. The controller 10 controls telephoning and image sensing according to user's selection. The telephone transmitter/receiver 20 transmits/receives a telephone signal under control of the controller 10. The image sensor module 30 senses an image under control of the controller 10.
As just described, the conventional CDMA mobile phone is equipped with the image sensor module 30, which is driven by supply of a clock signal. An explanation thereof will be given hereunder with reference to FIG. 2.
FIG. 2 is a view for explaining a clock signal of a conventional image sensor.
FIG. 2 illustrates an image sensor module (ISM) installed in the CDMA mobile phone. To operate the image sensor module (ISM), a sine-wave main clock MCLK signal generated by a clock signal generator (X-tal or TCXO) 11 is transferred to the CMOS image sensor and the image signal processing IC of the image sensor module 30 either directly or through a baseband IC.
Such a clock signal will be explained with reference to FIG. 3.
FIG. 3 is a frequency spectrum diagram illustrating the clock signal of the conventional image sensor module. In FIG. 3, the clock signal of the image sensor module includes a fundamental component and a plurality of harmonic components.
Meanwhile, examples of the clock signal include the main clock signal MCLK, a video clock signal PCLK and a I2C control clock signal SCLK.
The main clock MCLK signal is used to operate the CMOS image sensor and the image signal processing IC, and serves as a reference clock for the video clock PCLK. The main clock signal is generated from the TCXO or X-tal of the mobile phone either directly or through the baseband IC. Here, the main clock signal carries harmonic components besides a fundamental spectrum.
The video clock PCLK signal is used in synchronization of a video signal (RGB or YCbCr signal) which is fed through an actual sensor and the image signal processing IC. This video signal varies depending on size of video data. For example, the video signal varies according to the number of pixels (QCIF, QVGA or 2M), the frame rate and the chipset types.
Moreover, the I2C control clock SCLK signal is a synchronizing signal for controlling the image sensor module (ISM) independent from actual data.
Here, the main clock signal MCLK passes through a body of the image sensor module and a flexible printed circuit board (FPCB) and then the harmonic components of the main clock signal pass through air or the PCB and then are transmitted through an antenna of the CDMA mobile phone. The harmonic components transferred in this fashion are induced as noises into an actual cellular CDMA, GSM or next-generation communication frequency bandwidth.
A conventional method for overcoming such shortcomings will be explained hereunder.
Noises can be minimally radiated from the image sensor when both the telephone transmitter/receiver of the conventional CDMA mobile phone and the image sensor module are in an off state. To this end, noises are separated through a ground shielding, or the antenna and the image sensor module are physically spaced apart form each other at a greatest distance.
Alternatively, a low band pass filter is applied to the image sensor module to pass the fundamental component of the main clock MCLK and block the harmonic components.
As described above, the conventional method of reducing noises, i.e., harmonic components of the clock signal in the conventional CDMA mobile phone concerns utilizing the ground shielding or distancing the antenna from the image sensor module. However, such a method fails to sufficiently diminish the harmonic components of the clock signal of the image sensor module.
Furthermore, the low band pass filter is implemented through a passive device in practice, thereby significantly raising a device price and increasing filter size and costs.
In addition, a notch filter may be employed to remove the harmonic components from the clock signal. However, this fails to cancel noises outside a specified frequency band.